In reflective electron beam lithography, a dynamic pattern generator (DPG) is utilized to deliver exposure patterns. The DPG includes an array of mirror cells. In a process for patterning a wafer, some mirror cells in the array may absorb electrons, while the other mirror cells may reflect electrons back to the wafer. As the wafer size continues to increase, the dimension of the mirror array increases as well. DPGs are required to meet the high throughput requirement.